In the field of clock-making timepiece parts are assembled together by means of a glue or an adhesive. The gluing points must be formed by means of a repositionable hot melt adhesive to enable them to be reworked using heat in a manual operation and to allow the glued parts to be repositioned in an optimum manner either during assembly or during maintenance or repair of the parts as part of aftersales service. Moreover, this adhesive must also have resistance to chemical cleaning operations, in particular to withstand timepiece cleaning operations conducted during the aftersales service.
As an example, the ruby pallet stones are secured to the pallets by gluing, generally by means of a shellac. Shellac is a natural resin that is thermofusible, i.e. it has a honey-like consistency at temperatures higher than its melting temperature (Tf) and is solid below its melting temperature (Tf) or its glass transition temperature (Tg). Thus, shellac can be softened by an operator using heat so that the pallet-stones can be displaced a few microns on the pallets. However, like any hot melt compound, shellac is soluble in solvents. Parts assembled using shellac therefore have a poor resistance to timepiece cleaning operations.
Another example relates to the balance spring glued to its stud by means of thermosetting glues such as the resin Epikote™. Thermosetting glues have a good chemical resistance. However, they cannot be reworked using heat, and therefore parts assembled using thermosetting glues cannot be separated and repositioned if required.